Vacuum-tube.



W. D. COOLIDGE.

VACUUM TUBE.

APPLICATION FILED MAY 91 1913.

Patented 001. 31,1916.

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funun'uununununflnun Inventor William D.Coo||dge His Attorney k 20 27Witnesses 1" UNITED STATES PATENT OFFICE.

WILLIAM D. COOLIDGE, F SCHEN'EGTADY; NEW YORK, ASSIGHOB, T0 GENERALELECTRIC COMPANY, A. CORPOBATTON OF NEW YORK.

vacuum-Toma.

Specification of Letters Patent.

Application filed lay 9, 1913. serial No. 766,549.

To all whom it may concern:

Be it known that I, WILLIAM D. Goomnon,

' a citizen of the United States, residing at Schenectady, in the countyof Schenectady and State of New York,have invented certain new anduseful Improvements in Vacuum- Tubes, of which the following is aspecification.

My invention relates to vacuum tubes and more especially to tubesoperated for the purpose of producing Rontgen or X-rays. The tube, whichI have produced differs so radically from the tubes of the prior artused for producing Rontgen or .X-rays as to amount not so much to animprovement on prior tubes as to an entirely new variety of tubediffering .both in its principles of operation and in its operatingcharacteristics.

In the Rontgen tube of the prior art, it

was necessary that there be at least some free gas in the tube,thereason for which may be explained as follows: The number of ionspresent in the tube was exceedingly small until a high potential was imressed upon its terminals. The effect of th1s high potential was toaccelerate the motion of the small number of ions originally prese nt,thus causing them, by their coll1s1on with gas molecules to produce moreions, and,

further, to so attract the positive ions that they would bombard thecathode and so cause it to emit electrons. These electrons, directed bythe electric forces in the tube, move toward the anti-cathode and intheir movement constitute the so-called cathode rays. Upon collidingwith the anti-cathode these under- -go,a sudden change in velocity andso give rise to the Rontgen or X-rays. As the ennssion of electrons fromthe cathode was due solely to bombardment by positive ions, and as thesepositive ions owed their existence to the presence of gas in the tube,it is seen at once that the X-ray tube of the prior art was absolutelydependent for operativeness upon a proper amount of gas. Without thepresence of sufficient gas to permit a discharge to take place betweenthe electrodes the cathode would not be bombarded by positive ions andso no resulting production of X-rays'would occur. The presence of thisgas, heretofore considered a necessity in the operation of such a tube,gave rise to certain. features of operation of the tube which areobviously very disadvantageous but which- ,have heretofore been acceptedworkers in the art as unavoidable evils. Wlll not here attempt to pointout all of the troubles which may betraced to the presence of this gas,but will mention a few. of

the attendant diificulties.

(a) The pressure of the gas in such a tube is exceedingly variable, andany change in gas pressure is accompanied by a correspondmg change inthe resistance of the tube and in the penetrating power of the Patented0017.31, 1916. V

rected and, as a result, the exposure orv fluoroscopic examination ismade, not with Riintgen rays of the quality desired, but with whateverapproximation to that is allowed by the idiosyncrasies of the particulartube in question, in other words, with a mixture of rays differingperhaps very widely in penetrating power.

((1) The tendency to undergo rapid changes 1n gas pressure increaseswith use, until finally the tubes of the prior art show such instabilityof vacuum that they are no longer useful, even though they may beotherwise in perfect condition.

1 (6) Many of the positive ions instead of striking the cathode, strikethe glass constituting the tube wall around it, and, by their excessivelocal heating, cause disintegration and cracking of the envelop at thispoint. This imposes a limitation on the ermissible energy input in sucha tube.

Many tubes of the rior art show variability in the location 0 the focalspot. As the focal spot is the source of the X-ray, its movement duringa radiographic exposure, means a blurring of the picture. vIt

1s my belief that sudden changes in gas the cathode are impeded in theirpassage to cording from .duced.

As a cathode ,I ordinarily use a tang-- 'sten; filament and this may beconven-- V the target by impact 'with intervening gas molecules and sodo not have, upon reaching the target, the same velocity as those'electrons which have passed through the entire potential drop fromcathode to target without meeting an obstructlon. The higher the aspressure in the tube the greater will be t e mfluence of this effectand, consequently the greater will be the lack of uniformity in thepenetrating power ofvthe revsulting Riintgen rays.

eifect'm the radiograph and on the -fluores-.

cent screen.

(13) Any electromot1ve force 1n thesource of current supply for such atube, in a direction which would makethe target function.

as cathode, must be carefully suppressed, as it otherwise leads tochanges in gas ressure.

In order to overcome these trou les met with in the X-ray tubes of theprior art, I remove the cause of the trouble, namely,the

as in thetube, and produce the electrons om the cathode necessary to theproduction of the X-rays, not by a bombardment of the cathode bypositive ions due to ionization of the gas y the volta e impressed uponthe tube, but by heating ,t e cathode to incandescen'ce by energy .froma suitable source. In tubes made according to my invention I eliminatethe action of positive ions as far as it is possible to do so byevacuating the tube to the highest vacuum which can possibly be attainedand by thoroughly freeing the electrodes from gas. In the tube of theprior art the pressure of gas was quite appreciable and varied from sayone to ten microns (0.001 to 0.010 millimeters) of mercury. With a tubemade acto my invention, the tube proper and the electrodes being thorouhly freed s as hereinafter'describe the pres-' sure 0 as may be say fivehundredths of a micron 0.00005 millimeter) or lower and in any eventlower than that at which the tube would operate as an ordinary-Rontgenray.

tube. The upper limit of pressure may therefore be placed .rou hly atsay about 0.6 of a micron or therea out. .With such a vacuum in a tubeofithe prior art so far as I am aware no current whatever would pass inthe tube iently heated by current from a storage battery or transformerwith suitable regulating and no X-rays would be prodevices so that thetemperature of the cathode or filament may be adjusted at will. I havefound that electrons will be emitted from such a cathode, whiclbelectrons will traverse the space betweenthe cathode and the targetunder the influence of the electromotive force impressed upon the tube,and by bombardment of the target, will give rise to X-rays. In this newtype of tube positive those used in tubes ofthe prior art, the

tungsten filament, functioning as cathode,

is raised by these same discharges from room temperature to extremeincandescenoe, and may even be melted by the bombardment of the positiveions.

Gas pressure in the tube bein thus eliminated to the highest possiblefeatures ofinstability in the tu of the prior art which it can now beseen were due to the presence of the gas in the tube are likewiseeliminated. Furthermore, by static or electromagnetic means I cause thecathode rays or electrons to be focused on the anode or target and thisfocusing effect is steady and unwavering. I

A tube made in accordance with my invention differs from the'tubes ofthe rior art in the possession ofmost remar able characteristics. Thusthere are nomarked changes in the operation of the tube-during aconsiderable length of time, with the result that X-rays may beconstantly eneratedof a'much more uniform degree 0 penetrating power andmuch more constant in volume and in the location of their source orfocal spot. The penetrability or in'other words the hardness. orsoftness. of the X-rays may be adjusted at will by ad'usting thetemperature of the cathode. ere is practically no fluorescence of theglass of the tube and this is important because it indicates that theglass is not bombarded by target or anodeand is therefore not beinglocally heated thereby. There is only a such a'useless and disturbingfactor. in the ordinary Riintg'en tube.- The volta ge cone ee, thosesecondary cathode rays emanating from the sumed by the tube may incertain forms of embodiment of my invention bemade to renfii'n more orless constant even though the voltage of the source of current supplyvaries. The focusin efiect is steady and the 811832888 of definitlon ofthe X-ra photogra is increased. These and ot er features and advantagesof my invention will be better understood by reference to the followingdescription taken in connection with the accompanying drawings. Thefeatures of novelty I have endeavored to point out in the appendedclaims.

In the accompanying drawings, Figure 1 is a sectional view of a completetube equipped with one type of focusing device; Flgs. 2 to 7 illustratealternative forms of focusing devices, and Fig. 8 illustrates a modifiedtube having a constant voltage characteristic.

Referring to Fig. 1, showing one embodiment of my invention, the tube orenvelop is indicated at 1, consisting as usual of glass, or othertransparent material. In the tubular end 2 is sealed an anode 3 which inthis case is plate-shaped. It is supported by a stem 4, both anode andstem consisting, preferably, of wrought tungsten. Opposite this anode,which serves also as target, is a cathode 5 adapted to be heated,in-this case a filamentary spiral of tungsten, tantalum, carbon or otherhighly refractory material. In this case the filament is woundnon-inductively, though this is not essential. It is connected toleading-in wires 6, 7, sealed into a glass support 8, the wires beinginsulated from each other by little glass tubes 8' as clearly shown. Thewires 6, 7 are connected to a source of current, such as'a battery 9, orto a transformer, through a regulatable resistance 10. The battery, ortransformer.

secondary, should be well insulated owing to the high potential used inoperating the tube. Around the cathode proper is a hollow cylinder 11consisting of tungsten, aluminum, iron, or other suitable conductivematerial held in position by springs 12, one of which engages with alittle knob 13 on the glass. This cylinder or tube is preferablyelectrically connected to the cathode by means of wire 14, either insidethe tube or brought outside of the tube as shown, and containing incircuit a switch 15. In normal operation the cylinder 11 performs theimportant function of modifying the static field to focus the cathodedischarge, as will be explained.

Instead of being tubular the focusing device may assume various othershapes. It may be a perforated fiat disk, as shown at 16, Fig. 2, or aperforated dished or concave disk 17, Fig. 3, or even a simple ring 18,Fig. 4. In fact, even a metallic deposit, such as a film of chemicallydeposited silver, on the walls of the globe, as shown in Fig. 5 at 19,will act as a static focusing means.

Magnetic focusing means may be used, instead of the static focusingdevices, and as shown in Figs. 6 and The magnetic focusing means maytake the form of a solenoid, either inside the tube similarly to thestatic focusing ring, or outside the walls of the tube in thevicinity'of the cathode, as diagrammatically indicated in Fig. 6, at 20,or the cathode itself is wound inductively, as shown by cathode'21 inFig. 7. i

In the preparation of the tube the glass is first freed from moistureand gas by heating and other approved methods and the electrodespreferably are freed from gas as by heating them to a high temperaturein a vacuum. Exhaustion of the tube is preferably carried on by means ofa pump which will remove vapors as well as gases, such as the Gaedemolecular pump. For the removal of the last portions of gas from theelectrodes and from the glass the tube is operated on the pump. Theelectrodes should be made of refractory metal such as tungsten so thatthey will withstand the very high temperatures attainable by operatingthe tube while connected 'to the pump, whereby gas may be, effectivelydriven out. In case the usual constriction in the exhaust tube isemployed, pumping is thereby greatly delayed. In some cases much timecan be saved by omittinggthe constriction until the tube has beenexhausted to the point where the operation is satisfactory. Carefullydried air can then be admitted and the constriction put in. Uponexhaustion of the dry air it will be found that the condition of thetube, even without further running on the pump, is essentially what itwas before air was admitted.

lVhen the electrodes have been sufliciently freed from gas particularlythe anode which is of large mass relative to the cathode and the tubehas been exhausted to a vacuum of about .05 microns (0.00005 millimetersof mercury) or lower it is sealed off from the pump. It may be operated,in accordance with my invention, by closing the battery circuit throughthe resistance 10 so as to heat the filament 5 to incandescence. \Vhen asufficiently elevated temperature has been reached electrons are emittedby the filament. When a sufiicient potential is applied across the tubeby completing connection to a suitable source 22, through conductors 23and 24, current is transmitted, provided the negative terminal of thesource is connected to the filament. When the source 22 furnishesalternating current, as for example, a transformer or an induction coil,only half Waves of current are transmitted by the device. The electronssoon charge the hollow cylinder 11 to a negative potential with respectto the cathode unless the cylinder and cathode are connected. \Vhetherconnected or not, the cylinder somodifies the nature of the static fieldwithin the tube that the electrons are directed upon a spot of smallarea upon the anode 3.

. In my opinion the focusing action is caused by the electrons tendingtomove perpendicularly to the equipotential surfaces which may be drawn inthe space between cathode and anode. This manner of focusing the cathodedischarge is especlalllyeffective as the speed of the electrons is reatively low near the'cathode and therefore their movement is easilydirected by the focusing devices. The focal spot on the anode remainsfixed during the normal operation of the 'tube. This is in sharpcontrast to the operation of the ordinary Riintgen tube n which thefocal spot often moved; about so rapidly as to produce blurring of alllines not parallel to the direction of motion, even during very shortradiographic exposures. Z

The operation'of X-ray devices made in accordance with my invention-ismarkedly affected, by .the tem ature to which the cathode is raised. Ithe temperature of the filament is low only a small number of elec-'trons escape, and therefore the saturation current through the tube isrelatively small; Increasing the impressed voltage on the tube willcause substantially no further increase in current. It will increase thevelocity of the cathode Is 5 and hence the penetrating power-of the"ntgen rays. If the filament temperature on the other hand is very high,further increase of temperature produces no appreciable effect on thetube ,characteristics, more electrons being emitted by the filament thancanbe carried,

by the voltages emploiled, from one electrode to the other; At gherfilament temperatures, the space relations of the elements of the tubesuch as the distance between cathode and anode, the location of thefocusing means relative to the cathode, the diameter of the opening inthe focusing device, etc. constitutes a current hmiting fac-.

tor, as will be later e lained. Except for these limiting cases, anincreasev in impressed voltage will in general cause an increase 1ndischarge current and in the potential drop in the tube, thus increasingthe penetrability of the X-ray. However, by giving the cathode its mainextension in the direction of the axis of the tube, and also suitablyproportioning and locating the focusing device, a most important newcharacteristic may be attained. Such a construction is shown in Fig. 8in which a filament 25 extends nearly through the whole length oftube26, being'anchored at its tip by a support 27. With a low impressedvoltage, only that part of the filament 25 nearest the anode willfurnish the electrons necessary to-carry the discharge current. Withhigher impressed voltages, a progressively greater part of the filamentwill contributeelectrons. The increase in the number or electrons thusrendered available by the increase of impressed voltage and theaccompanying change in distribution in the static field, tends to reducethe tube resistance and thus counteract the tendency of the higherinduction coil or a rectifying device such as a mechamcal selector areuse d which gives current impulses of variable voltage.

My novel apparatus therefore, has the following valua lo and strikingcharacteristics. First, the resistance of the tube, and, hence thepenetrating power of the Riintgen rays produced in the tube, may becontrolled with certainty, and may be instantly increased or decreasedat will, by raising or lowering of the temperature of the cathode;

second, a much more homogeneous'bundle' of X-rays can'be produced eventhough a several-fold increase, or decrease of as pressure takes placeinthetube, providing the pressure remains within the limits aboveindicated; third, as the -dischar e canpass through the tube only whenthe eated electrode is made the cathode, the tube may be operated onalternating" current almost equally as well as on direct current;fourth, if desired, a tube may be proportioned to give over apredetermined range of impressed voltage X-rays of substantially uniformpenetration.

The operator is thus enabled to determine, with a properly calibrateddevice,

from a curve, or chart, just what current values and voltages he needsfor a given purpose, and may be sure that when he has prepared hispatient he can get exactly the service he requires from his tube withouttroublesome and dangerous experimentation.

One of the main causes for the uniformity of the penetrability of theX-rays is to be ascribed to the fact that the operation of the tubeisnot dependent uponv ositive ion bombardment of the cathode. othercause is its complete independence of vacuumchanges within thepermissible range of pressure. It will also be noted that with the 'highvacuum existing in the tube, the mean free path between gas molecules ismuch greater than the distance between anode and cathode and, therefore,very few electrons can collide with gas molecules in.

passing from cathode to anode, therefore,

-practically all of the electrons reach the target with uniform velocitBecause of the low gas pressure the num er of positive ions is'so smallthat the filament is not heated at all by bombardment. Even when a veryheavy discharge current is sent through the tube, a voltmeter andamineter in the filament circuit indicate no change in the resistanceand hence the temperature of the filament. Another observation, 'whichhas a similar significance, is that at low filament temperatures, wherethe electrons emitted by the cathode are allused in carrying current, aseveral-fold increase in voltage brings substantially no accompanyingincrease in the discharge current. This could hardly pertain, in casepositive ions played an appreciable role, for increased voltage wouldmean higher velocities, and hence, a greater emission of electronswhich, in turn, would mean an increase in the discharge current.

The wall of the tube exposed to the anode is not heated by secondarycathode rays from the anode, thereby removing the limitation formerlyimposed by local tube heating. Tube fluorescence due to secondarycathode rays is likewise absent. Not only are current carryinglimitations thus removed but it should also be noted that a marked gainof efficiency is secured in the tube. For example, in the former type oftube with a platinum target, about threefourths as many electrons leftthe target as secondary cathode rays as were received by it in the formof primary cathode rays. The elimination of secondary cathode rays alsogreatly improves the tube as the secondary X-rays produced by thesecondary cathode rays striking the tube were a serious disturbingfactor as already mentioned.

While I have described and illustrated specific forms of tubes embodyingmy invention, I wish it to be understood that various changes andmodifications may be made in the tube within the scope of my invention.As already indicated the spacial relations of the elements of the tubeaffect its characteristics. For example, increasing the distance betweenthe front of the focus-' ing device and the incandescent cathodeincreases the resistance of the tube; decreasing the opening in thestatic focusing ring or tube increases the resistance of the tube.

Decreasing the distance between cathode and anode lowers the resistanceof the tube, but this effect is probably not to be ascribed to anyconsiderable resistance change due to linear separationof the electrodesbut to the fact that the glass walls of the tube are charged negativelyduring operation and thus act as though they represented an extension ofthe focusin device. Bringing the cathode nearer to t e. anode therefore,would have an effect similar to that caused by bringing it nearer to thefront of the focusing tube or ring.

Certain broad aspects of the apparatus and methods herein disclosed arenot of my invention but are the invention of Irvin Langmuir and aredefined by the claims 0 his allowed application, Serial No. 84 t2,renewed March 14, 1916, and his a p 'cationi Serial No. 876,432, filedDecem er 10, 191

. What I desire to secure by Letters Patent of the United States, is

1. In a device for the production of X- rays, the combination of a tubeprovided with electrodes, the vacuum in the tube being soattenuated aspractically to'eliminate the production of positive ions, and means forgenerating electrons at the'cathode or negative electrode substantiallyindependently of the voltage impressed upon said electrodes.

2. An X-ray device comprising a gastight envelop, the space within saidenvelop. being exhausted to a pressure at which substantially no gasionization occurs, a cathode, an anode, and means for generatingelectrons at the cathode, the current transmitted by said device with agiven cathode temperature being independent'of the voltage over a rangeof impressed voltage values.

3. An electrical vacuum discharge tube, comprising an envelop,cotiperatin electrodes, at least part of one of w ich is adapted to bemaintained at incandescence, the vacuum within said envelop being sohigh that evidences of positive ionization are substantially absentduring operation and means for focusing the discharge.

4. An electrical vacuum discharge tube, comprising an envelop,cooperating electrodes therein, one of which is adapted to beindependently heated, and a conducting body surrounding said cathode,said body serving to. modify electrostatic forces in the tube, tocontrol the direction of the cathode rays, the residual gas in saidenvelop being insuflicient to produce appreciable ionization bycollision during the operation of the tube;

5. An X-ray device com rising electrodes consisting of tungsten reedthoroughly from gas, the negative electrode being adapted to beindependently heated, at least in part, and an inclosing envelop sohighly evacuated that evidences of positive ionization are substantiallyabsent during the operation of the device.

6. An electrical discharge device com rising an envelop, the spacewithin sai envelop being evacuated to a pressure below the value atwhichappreciable positive ionization can occur, electrodes therefor, the

cathode being adapted to be heated inde-' pendentlyof energy transmittedby said device and conductive means in part at least surrounding the sace between said' electrodes to control e ectrostatic forces in thetube.I

I 7. An electrical discharge device compristobe'heated independentlyof-the operating current, a cooperating anode and means for focusing thecathode discharge upon the anode. e

9. A Rontgen ray device comprising the combination of an envelopevacuated to a pressure at which positive ionization is substantiallyabsent; during the operation of the device, an electron-emittingcathode, and anode and a source of ener connected to said electrodeshaving a vo tage in excess of the minimum value at which the current issubstantially independent of the voltage.

10. A Riintgen apparatus comprising an envelop evacuated to a pressureso low that evidences of positive ionization are substan tially absentduring the operation of the apparatus, electrodes therein, externalmeans for independentlyheating the negative elec-- trcde,'and means formodifying the static field in theneighborhood of the cathode to focusthescathode discharge upon a spot of small area on the anode. p

11. A Riintgen tube comprising an evacuated envelop, main workingelectrodes there- I for, one of which is offilamentary character, meansfor heating said filamentary electrode, and an electrical conductor atleast in part surrounding the space between said electrodes .forconcentrating a discharge from said cathode upon a spot of smallarea'onthe anode.

12. A Rontgen ray tube comprising an evacuated -envelop,' electrodestherein, thecathode being arranged [to be heated indedently, and afocusing device surroundmg and electrical] connected with said in-'dependently heate cathode.

13. The combination of a source of current alternating in potential,'and a Rfintgenray tube operatively connected therewith comp cooperatingelectrodes at least part of one of which is adapted to independentlygenerate electrons, and an envelop therefor evacuated to such highdegree that manifestations of positive ions are absent.

14. A Rdntgen ray tube comprising an envelop, cotiperating electrodestherein, atleast-part of one of which may be independ-' en'tly heated toemit electrons, the space in said'envelop being so highly evacuated thatmanifestations of positive ions are a .means for-producing a cathodedischarge in evacuated to such a degree that bsent,

tive ions are absent, a source of high potential current,connectionsbetween said source and said main electrodes, an independentsource of low potential current connected to and'heating the cathode,and means for adjusting the heating current to control the resistance ofthe X-ray device.

16. In a device for the production of Riintgen rays, the combination ofa tube practically no current can pass due to ionizing action of theimpressed voltage upon such gases as may remain in the tube, s'itive andnega-' tive electrodes in the tu pr: "ided. with suitableleading-in'conductors, and means for heating the cathode to a hightemperature, said means being independent of the impressed voltage.

17. An electrical vacuum tube, comprisin an envelop the space in which1s exhauste to below the pressure at which residual gas will conductcurrent, a cathode adapted to" be heated to incandescence, acoiiperating anode substantially deprived of gas evoluble by electronbombardment, and means 'disv posed about an axis passing through cathodeand anode for directing the, cathode discharge. v

18. An electrical discharge device-comprising an envelop evacuated to apressure so low that evidences of positive ionization are substantiallyabsent during ,the o ration'of'the device, an anode, a cathode a apt edto be heated independently of a dischar passing between the same and theano e, i

and conductive means surrounding, in part at least, the space betweencathode and anode for modifying the static field to focus a cathodedischarge. I

19. The method of producing Riintgen rays which consists in independentlheatmg ucing an thecathode to incandescence, p electric dischargebetween the incandescent cathode and a cooperating anode in a vacuum sohighly attenuated that evidences of positive ionization aresubstantially absent during the passage of current, and focusinghtladcathcde rays by means of an electrica e 4,

'20. The'method of regulating thepenetrability of Rontgen rays producedby a discharge from an incandescent cathode in a vacuum so. attenuatedthat electrical conductivitykof the residual gas is substantiallyabsent, whidl consists in adjusting the cathode temperature and impvoltage.

v I. I

21. The process .of producing Rontgen rays by current from a. sourcealternatmg in potential which consists in emitting electrons at anegative electrode, transmitting 6 thereby a ray-producing current waveof one polarity, while preventing at the same time the formation ofpositive ions, and suppressing the other half .wave of the alternatingcurrent.

which consists in impressing an alternating electromotive force uponelectrodes of an X-ray tube and causing electrons to beemitted from oneelectrode only while preventing any accompanyingmanifestations ofpositive ions, and pro ectin said electrons upon a target surface, wherey one-half wave of an alternating current is utilized and the oppositehalf wave suppressed.

23: The method of operating a tube for the production of X-rays whichconsists in maintaining within the tube a vacuum so highly attenuatedthat practically no current can flow through the tube under theinfluence of the impressed voltage through the medium of ionization ofgas molecules in the tube, and rendering the tube unilaterallyconducting by generating electrons at the cathode independently of thevoltage impressed upon the tube.

24. The method of operating a tube for the production of X-rays whichconsists in maintaining within the tube a vacuum so highly attenuatedthat practically no our-- rent can flow under the influence of thevoltage impressed upon the tube, through the medium of ionization of gasmolecules in the tube,and rendering the tube unilaterally conducting bygenerating electrons at the cathode by current independent of thevoltage impressed upon the tube. 25. The method of depriving a metalbody of occluded gas which consists in gen erating electronsconstituting the origin of an electrical discharge, independently of thevoltage producing the .discharge, in an evacuated space, the residualgas in which is so attenuated that at the given voltage the gas will notconduct'the discharge without the inde ndent generation of electrons,subjecting t e metal body to the electron discharge, and coincidentallyremoving gas evolved from said metal body to maintain the gas pressureso-low. that at the given voltage a discharge will not occur withouttheindependent neration of electrons.

26. The metho of depriving a' metalbody of gas\which 1consists .ingenerating -elec trons constituting the origin of an electric dischargeindependently of a voltage producing the discharge, in an' evacuatedspace in which the residual gas. pressure is so low that at a gsvcenvoltage the 'gas will not conduct the harge without the independent 65generation 22. The process of producing X-rays of electrons, subjectingthe metal body to the electron discharge, the energy. of. thedischarge.being suflicient to heat said metal body to incandescence, and

coincidentally removing gas disengaged by. said discharge to maintainthe gas pressure solow that at the given voltage a discharge will notoccur without the independent generation of electrons.

27. The method of preparing a discharge device, comprising an envelop,-an anode and 76-. a cathode which consists in generating electrons atthe cathode independently of the voltage producing a discharge from saidcathode, the space. in the envelop being evacuated so completely that noenergy can 80 be passed at a given voltage between said electrodeswithout the independent generation of electrons at the cathode, removinggas liberated by said discharge and finally sealing the envelop when theresidual gas pressure is so low that the device can be operated withoutappreciable evidence of positive ionization.

28. The method of depriving a metal body of gas which consists inheating a cathode to incandescence to generate electrons, assing betweensaid cathode and said ody used as anode an electrical discharge in avacuum so high that at the discharge voltage the residual gas will notconduct current without the independent generation of electrons at thecathode and coincidentally removing gas evolved from said metal body tomaintain the gas pressure so low that at the given voltage a dischargewill not occur 1 without the independent generation of electrons. p

29. The method of preparing an X-ray device comprising an envelop, ananode and a-cathode, both consisting of refractory ma-. 1 terial, whichconsists in heating the cathode to incandescence to generate electrons,passin between said cathode and anode a high vo tage electricaldischargeof suflicient energy to heat the anode in a vacuuin so high thatwithvthe given voltage the discharge is not maintainable while thecathode is unheated, removing gas from the evacuated space liberatedfrom the anode by said dis-' charge and finally'sealing the envelop whenthe residual gas pressure is so low that the device can be operatedwithout appreciable evidence of positive ionization;

30. The process of operating an X-ray device comprising a gas-tightevacuated envelop, a cathode and an anode, which consists in generatingelectrons at the cathode independently of current passed by said devicewhile preventing any accompanying manifestations of positive ions,impressing on the electrodes a voltage at least as hifgh as the value-atwhich substantially all 0 the electrons are utilized, whereby thecurrent becomes substantially independent of the. impressed voltage, andincreasing or X-rays b respectively 1ncreasing or decreasing t eimpressed voltage.

tially absent durin 31. An X-ray device comprising a gastight envelo anelectron emittin cathode, and an ano e deprived of-ionizab e gas, thespace in said envelo being evacuated to a pressure at least as ow asfive hundredths of a micron of mercury pressure, the current transmittedby said device with a given electron-emissivity of the cathode beinsubstantially constant with an impress voltagevariable over a workingrange.

32. A Rontgen ray device comprising a sealed enve1op,"the space withinwhich is evacuated to a pressure solow that evidences of positiveionization are substanthe operation of said device, a cathode orefractory metaliadapted to be heated substantially independentlyof avoltageimpressed upon said electrodes and a 006 rating, anode.

33. An -ray tube comprising a sealed envelo a cathode adapted to beheated indepen ently of a discharge transmittedby said device, 'an anodeand a dished focusing member surrounding the' cathode having its concaveside facing the anode, said tube being exhausted of gas to a pressure solow that evidences of positive ionization are substantially absent durinoperation.

In witness whereof, have hereunto set my hand this 8th day of May,191-3.

' V D. 'COOLIDGE.

Witnesses: Bmmum B. HULL,

